1. Field of the Invention
This invention relates to textured coatings for metal articles. In particular, it relates to polyester-based coatings which offer the appearance of a striated poly(vinyl chloride) (PVC) plastisol-based coating. More particularly, this invention relates to coil coating compositions which give a striated pattern simulating a wood grain on aluminum siding.
Thixotropy is the property of a liquid whereby its viscosity decreases as it is stirred at a constant shear rate but increases again to its original value some time after the shear rate is reduced to zero. In the coil coating operation, a coil of sheet metal is uncoiled as it is pulled through a series of rollers, one or more of which is a paint applicator roller, at up to 600 feet per minute. It is then passed through a curing oven and coiled again for the market. The paint is picked up by a roller rotating in the paint pan and transferred to an applicator roller. The thixotropy that is necessary for the formation and retention of the striations must be balanced against the need for recirculation of the paint from the paint pan to a drum from which it was pumped into the pan.
2. Description of Related Art
Existing technology for the creation of a striated appearance on sheet metal on a single pass through a coil coating apparatus is limited to the use of PVC plastisols. The film thickness of said plastisols must be from 3 to 5 mils in order to give proper coverage to the substrate and to produce the desired striated pattern. The cost of such a thick film per square foot is relatively high when compared with other coatings. Also, the poor heat stability of PVC resins is a problem during the manufacture of the plastisol and during its use on the coil coating line. The heat sensitivity can lead to gelation of the plastisol which makes it unusable.